Shadow mask blank for color cathode ray tube

ABSTRACT

The aperture mask of a color cathode-ray tube is formed of a blank of annealed steel which has a multiplicity of apertures disposed in a rectangular field. The opposed surfaces of the blank have etchant resistant oxide layers and the walls of the apertures are lined with a flash coating of a material which is resistant to oxidation but is subject to attack by the same etchant that attacks the material of the blank. The apertured blank is used for photographic printing of the screen of the color tube after which it is subject to an etch bath which first removes the liner and then, by attacking the walls of the apertures, enlarges them to a size exceeding the phosphor deposits of the screen. With the apertures enlarged, and with the oxide layers retained, the mask is installed in operative position relative to the screen.

United States Patent [191 Kaplan Apr. 16, 1974 SHADOW MASK BLANK FOR COLOR CATHODE RAY TUBE [75] Inventor: Sam H. Kaplan, Chicago, Ill.

[73] Assignee: Zenith Radio Corporation, Chicago,

Ill.

[22] Filed: Oct. 15, 1969 [21] Appl. No.: 866,694

Primary Examiner-Robert Segal Attorney, Agent, or Firm-Nicholas A. Camasto; Francis W. Crotty [57] ABSTRACT The aperture mask of a color cathode-ray tube is formed of a blank of annealed steel which has a multiplicity of apertures disposed in a rectangular field. The opposed surfaces of the blank have etchant resistant oxide layers and the walls of the apertures are lined with a flash coating of a material which is resistant to oxidation but is subject to attack by the same etchant that attacks the material of the blank. The apertured blank is used for photographic printing of the screen of the color tube after which it is subject to an etch bath which first removes the liner and then, by attacking the walls of the apertures, enlarges them to a size exceeding the phosphor deposits of the screen. With the apertures enlarged, and with the oxide layers retained, the mask is installed in operative position relative to the screen.

6 Claims, 5 Drawing Figures SHADOW MASK BLANK FOR COLOR CATHODE RAY TUBE RELATED APPLICATION The mask in its preferred form is a similar structure to that described and claimed in copending application, Ser. No. 811,318, filed Mar. 28, 1969, in the name of Sam H. Kaplan and assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION Both the invention of this application and that of the related copending application concern the important black surround development of shadow-mask color television tubes described and claimed in U.S. Pat No. 3,146,368, issued on Aug. 25, 1964, and assigned to the assignee of the present invention. The principle of black surround is applicable whether the phosphor dots comprising the screen of the tube take the form of hexagons, dots or the like. The configuration of the phosphor deposits is of no particular consequence except that, in photographic screening, it imposes restrictions on the shadow mask since the phosphor deposits will have the same configuration as the apertures of the mask. In the usual case, the mask has a field of generally circular apertures to form dot deposits of phosphor and the invention will be described in that connection simply for convenience.

One significant difference in a shadow-mask tube having dot triad arrangements of different phosphors without black surround, as compared to the same tube having black surround,- is in the size relation of the apertures of the mask to the phosphor dots. Conventionally, that is to say, in prior art tubes lacking the principle of black surround, the dot deposits are larger in diameter than the electron beams and their size differential provides a tolerance or guard band for preserving white field purity. The black surround tube, however, has the reverse relation in that essentially the same size differential is maintained but now the electron beam is larger and the phosphor dot is smaller. This, of course, introduces difficulties in screening and a solution that has met with commercial success utilizes an aperture mask structure of the type described and claimed in the aforeidentified copending application. The apertures of that mask, in the arrangement illustrated in the drawing of the application, have a small opening which is the initial effective diameter of the apertures and .is dimensioned as required to achieve a desired phosphor dot size. Each such small opening is surrounded by a larger opening that is only partially etched through the mask, being concentric with the first described small opening and being in connection therewith through a wall section that is a small fraction of the thickness dimension of the mask. After such a mask has served its purpose in screening, it is introduced into an etch bath and etched for a second time. In this step, the small opening is enlarged so that the final aperture dimension is that necessary for achievinga desired electron beam diameter. An obvious advantage of this structure is that the reduced wall section surrounding the small aperture used in screening permits the second etch, or what is referred to as the re-etch step, to be accomplished in a short period of time. As a consequence, the flat surface of the mask blank are exposed to the etchant for a short period of time so that only a limited surface area of the blank is removed. This pennits the structure to retain its necessary mechanical strength.

The present invention is a stil further improvement over that of the copending application, achieving reetch with minimal reduction in blank thickness and retaining maximum mechanical strength of the mask. As

will be made clear in the following description, in its preferred embodiment, the invention also effects attractive savings in the processing steps of the mask with resulting economies.

Accordingly, it is an object of the invention to provide a novel shadow-mask structure for a color picture tube which makes it possible an improved process for fabricating such a tube.

It is a specific object of the invention to provide an improved shadow-mask structure, and an improved screening process utilizing such a structure in fabricating a color cathode-ray tube of the black-surround variety.

It is yet another object of the invention to provide an improved structure and process for fabricating a color cathode-ray tube, such as a shadow-mask or post-deflection-acceleration tube, characterized by an electron beam size which exceeds the size of the elemental phosphor deposits of the screen.

SUMMARY OF THE INVENTION A shadow mask for a color cathode-ray tube, constructed in accordance with the invention, comprises a mask blank of a material that is subject to attack by a predetermined etchant. The blank has a multiplicity of apertures formed therein and distributed in accordance with a prescribed pattern and further has on opposed surfaces thereof a coating of material that is resistant to that etchant. A lining covers the wall surfaces of the apertures in the blank to a thickness that is negligible compared to the smallest dimension of the apertures and is formed of material that is readily removed chemically, preferably being subject to attack by the same etchant.

In a preferred embodiment, the mask blank is formed of a ferrous metal that oxidizes readily and is opposed surfaces are provided with a layer of oxide. The apertures of the blank are lined with a flash coating of a non-ferrous material such as copper, nickel or the like.

The process of fabricating a color tube with such a mask comprises as a first step photographically printing on the screen surface and through the aperture pattern of the mask a plurality of interlaced deposits of different phosphor materials individually defining a phosphor pattern corresponding to the aperture pattern of the mask, or color-selection electrode as it is frequent'ly termed. Thereafter, the mask is subjected to a chemical milling treatment of sufficient concentration and for a sufficient processing time that the lining material is first removed from the walls of the apertures and, if this is accomplished by a r'e-etching step, the etching step is continued to the end that the apertures are enlarged. Preferably, they are enlarged to a size exceeding that of the deposits of the individual phosphor patterns.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the present invention which are believed to be novel are setforth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identifying like elements, and in which:

FIG. 1 is a fragmentary view of the screen section of a shadow-mask type of color tube and also a shadow mask positioned relative thereto in a manner required both for screening and for operation of the completed tube;

FIGS. 2-5, inclusive, are enlarged views of fragmentary sections of the aperture mask as it appears in dif ferent stages of the mask forming and screen forming processes.

DESCRIPTION OF THE PREFERRED EMBODIMENT Color tubes of the type under consideration may have circular or rectangular envelope configurations at the faceplate or screen section which includes the image area. In this area there is a repeating sequence of phosphor deposits which, for the tri-color system currently in use, represent the three primary colors employed in the additive type of color system. As stated above, the phosphor deposits may take any of a variety of forms but it will be assumed for convenience that the tube under consideration is a rectangular one having a mosaic screen defined by a multiplicity of dot triads each of which constitutes a dot of green, a dot of blue and a dot of red phosphor.

In FIG. 1, the screen 9 has phosphor deposits represented by circles and the legends G, B and R designate those of the deposits which are formed of green, blue and red phosphors, respectively. It will be observed that the phosphor dots are not in tangential contact as in conventional prior art shadow-mask color tubes that feature neither black surround nor post-deflection acceleration or focus. The geometrical relation represented in the figure indicates the phosphor dots to be smaller than usual and, therefore, separated from one another to provide an area of the screen around each such dot which may accommodate a light-absorbing material or pigment in accordance with the teachings of US. Pat. No. 3,146,368, although for convenience of illustration, that pigment has not been represented in the drawing. It will be recognized that this size relation is also employed in a post-deflection-focus tube whether it employs such a pigment surrounding the phosphor dots or not.

Superposed over and in close spaced relation to the screen or image area of the tube is a shadow mask 10, only a portion of which appears in FIG. 1. It is a relatively thin blank of material having a multiplicity of apertures 11 formed therein and distributed in accordance with a prescribed pattern of distribution. For the rectangular tube structure under consideration, apertures 11 are distributed throughout a rectangular field corresponding in dimension to the screen or image area of the tube. When the screen is deposited on the faceplate of the tube, which is customary, the mask is generally a spherical section so as to be nearly parallel to the screen surface.

It is well known and will become apparent as this description proceeds that it is necessary to arrange that the mask be removably installed in operative position within the tube envelope, in a position in which it is approximately in spaced parallel relation to the screen with each aperture of the mask essentially aligned with an associated dot triad of the screen as indicated in the representation of FIG. 1. Mechanically, this is readily accomplished by constructing the mask as a subassembly in which the mask is annexed to a frame which lends mechanical strength and facilitates accommodating supporting provisions. The supports generally are leaf springs affixed at one end to the frame and shaped so that an aperture at the free end thereof may engage a mounting stud projecting inwardly of the tube envelope. The tube envelope is a two-piece affair, having a conical and neck section as one major component and a faceplate section at the other. The faceplate is dish shaped and its flange accommodates the mounting studs which enter the openings of the leaf springs as the mask subassembly is installed in position. All of this structure is well known and constitutes no part of the present invention. Therefore, it has not been shown in the drawing.

It is appropriate now to consider in more detail the structure of mask 10 and the contribution it makes in facilitating screening of the type tube under discussion. The mask is formed from a sheet or blank of a material that is subject to attack by a predetermined etchant simply because chemical milling techniques, such as etching, for developing a desired pattern of holes in a blank of material that may be etched is a highly perfected art which makes possible the development of holes in precise locations and with accurately controlled dimensions. Such material as cold rolled steel, aluminum clad steel and the like may be employed or if desired, a copper nickel alloy is acceptable. Commercially, masks of steel are generally used having a thickness of 6 to 7 mils. In order to develop a desired aperture pattern there is applied over each of the opposing faces or surfaces of the mask blank a layer 11, 11a of a photosensitive resist, such as dichromated polyvinyl alcohol, fish glue or shellac. It is desirable that the resist have the property that it is normally soluble in a particular solvent, preferably water, but becomes insoluble when exposed to actinic energy such as ultraviolet light. A pattern, representing the aperture pattern desired, is then projected onto layers 11, 11a to create latent images of the apertures to be formed and these images are then developed by washing the resist coated blank with a solvent for the resist. More particularly, the pattern projected on layer 11 exposes all portions thereof except for those elemental areas 12 where an aperture is to be developed. In like fashion all portions of layer 11a, other than areas which are in coaxial alignment with areas 12, are exposed and the mask blank is then washed with the solvent. The developing step removes the resist from elemental areas 12 of photoresist layer 11 and elemental areas 12a of resist layer 1 l, leaving the resist on those portions of the blank surfaces that have, in fact, been exposed.

The blank is now immersed in an etch bath having an etchant, such as ferric chloride, which attacks the exposed elemental areas of the blank, that is to say, those portions of the blank exposed byopenings 12, 12a of resist layers 11, 11a. The etching time is, of course, a function of the etching agent, its concentration in the bath and the temperature. It may be determined empirically for any given etching process desired. Representative parameters for the etch are: a concentration of 45 Baume, a temperature of about F. and an etch time of about 1 minute. As a consequence of the etch, a pattern of apertures is formed in mask blank 10 but the portions of resist layers 11, 11a previously exposed to light remain in tact. With reference to FIG. 3, it is clear that each etched aperture has a large dimension D on one surface of the mask blank and a smaller concentric dimension D on the opposite surface. The mask portion between these openings is generally dish shaped providing a very much reduced thickness of mask material immediately contiguous to the small opening D in accordance with the teachings of the aforeidentified copending application. For the particular structure under consideration, the holes are generally circular and the dimensions D D are diameters. The apertures may be of uniform dimension but it is common practice to grade them, reducing the diameter with radial distance from the center of the mask. Illus tratively the hole size may range from 8 mils at the center to 7 mils at the outer edge of the mask.

Each aperture formed in blank now has its walls covered with a lining 13 which has a thickness that is negligible compared to the smallest dimension of the aperture. For theusual shadow mask where the apertures, at least in the center of the mask, have an effective initial diameter D of about 8 mils, the thickness of lining 13 is approximately 0.2 of a mi]. It may becharacterized as a flash coating and is formed of a material which is readily removed chemically, preferably being attacked by the etchant used in developing the apertures in mask blank 10. Nickel or copper is a suitable liner and may be applied byany conventional electrical nickel chloride 30 g/l sodium hypophosphate 10 g/l ammonium citrate 65 g/l ammonium chloride 50 g/l (solution is adjusted to a pH of 8-l0.)

Such a solution deposits a non porous coating of nickel and nickel phosphate on metals and alloys. Copper and cobalt may be applied in much the same way. During the coating process by which flash lining 13 is applied, the residual resist layers 11, 11a protect or shield the flat opposed surfaces of mask blank 10 to assure that only the walls of the mask apertures receive liner l3. Liner 13 is formed of a non-ferrous material so as to be resistant to oxidation for a purpose presently to be made clear. After liner 13 has been deposited, resist layers 11 and 11a will have served their purpose and they then are stripped by washing with a caustic solution, such as sodium hydroxide. While the caustic bath removes the resist layers, it has no effect on liner 13. It should be noted, in passing, that the etching step which forms the individual apertures of mask blank 10 may very well undercut resist layers 11, 110 but that is of no particular consequence to the invention and has not been indicated in the drawing.

In accordance with the teachings of this invention, it is most desirable that the opposed surfaces of blank 10 be provided with a coating of material that is resistant to the etchant used in processing the blank so that the mask apertures may be enlarged without reducing the blank thickness. A number'of materials may be used to coat the blank which are resistant to ferric chloride and similar etchants. For example, a layer of gold would suffice and other suitable materils are chromium and platinum. A preferred approach, however, both from the standpoint of economics and simplicity of processing steps, provides oxide layers 14 and 14a (see FIG. 4) on the opposing surfaces of the blank since it has been found that oxides are resistant to the etchant normally employed in developing the aperture pattern of the mask. This is attractive because oxidizing the steel mask blank is a process step that is otherwise normally undertaken in the fabrication of a color picture tube in order that the mask may exhibit the heat-radiating properties of a black body to prevent the mask from getting too hot due to heat generated by impinging electrons. Accordingly, it will be assumed that layers 14, 14a are achieved by oxidation of the steel mask blank in any known process which develops a firm, strongly adherent oxide layer, as distinguished from a flaky one, so that it endures, once it has been established, throughout the screening process and provides a suitable permanent coatingfor the mask as installed in the tube. The application of layers 14, 14a is often referred to as thermal blackening and may be accomplished by heating the mask to a temperature of about 1,040 P. in an oxidizing atmosphere. It may also be applied chemically by use of a salt bath or by high tem perature oxidation in a steam atmosphere; the so-called steam homo process. The mask having the properties illustrated and described in connection with FIG. 4 is suitable for screening the tube in process.

It is preferred that photographic printing be employed to form a plurality of interlaced deposits of dif ferent phosphor materials individually defining a phosphor pattern corresponding to the aperture pattern of mask 10. In photographic screening, as now well understood in the art, the screen is first covered with a layer of a resist that is rendered insoluble by exposure to ultraviolet light, polyvinyl alcohol sensitized with ammonium dichromate normally being employed. This resist also has as an ingredient one of the three phosphors to be applied. After the resist including the phosphor is applied over the screen area, the mask is installed in position and the screen in exposed through the apertures of the mask from a light source positioned to simulate the electron gun of the tube assigned to excite the particular phosphor in process. After exposure, latent images of the desired deposits of the particular phosphor will have been made and the mask is removed from its operative position in relation to the screen to facilitate washing the screen with a solvent, such as water, to develop such images and create deposits of the particular phosphor properly located on the screen area. These deposits will be essentially the same size as, or slightly larger than, the effective diameter D of the individual mask apertures.

In similar fashion deposits of the remaining two phosphors are made but, since the deposits are to be spaced from one another, it is essential that each step adopt a uniquely different position of the exciting light source. The screening with phosphors, insofar as a black surround tube is concerned, may take place either before or after the application of the black surround material as explained in Pat. No. 3,146,368.. Since that step is of no concern to the present invention, it will not-be considered further.

After the third set of phosphors has been deposited, the screen will have received its desired pattern of dot triads and the mask will have served its purpose in screening and is now ready to be prepared for final installation in the tube. More particularly, the apertures of the mask are to be enlarged to the end that the electron beams of the finished tube will have diameters, as determined by the apertures of the mask, which exceed the size of the deposits of the individual phosphor patterns.

For this purpose, the mask is removed from the faceplate section of the tube and is re-etched, that is to say, is immersed in an etch bath. Since liner 13 is preferably formed of material that yields to the etchant, liner 13 is first removed in the re-etching step. But the reetching is then continued to enlarge the apertures to a final desired effective diameter D as indicated in FIG. 5. This figure shows a section of the mask with the aperture of final size, larger than the individual phosphor dots, and also bearing oxide layers 14, 14a. This is the structure required for installation in the tube. It has both the proper effective aperture size and, by reason of the oxide layers, it has the heat-radiating properties of a black body. In the re-etch step, portions of oxide layers 14, 14a are concurrently removed as the apertures are enlarged simply because enlarging the apertures removes the support or foundation under the overlying portions of layers 14, 14a and they, consequently, disintegrate or disappear as the apertures enlarge as shown in FIG. 5.

If preferred or if liner 13 is not readily attacked by the etchant used for re-etch, the mask may first be treated as by spraying, washing or immersing into a chemical solvent or etch for the material of the liner. A commercially available etchant for stripping a copper liner is Enstrip-C. A nickel liner may be readily removed by reverse or de-plating in which the mask serves as an anode. In either case where the liner is separately removed, the mask is then re-etched in the usual way to enlarge the mask apertures to the size desired.

Of course, as the screen is being prepared in the described manner for final installation in the tube, the necessary final processing steps will also take place relative to the screen. These include filming and then application of the backing conductive layer of aluminum.

One simplification in the described arrangement is the omission of liner 13 which is the described embodiment of the invention serves the necessary function of introducing discrimination in portions of the mask structure in respect of oxidizing. The desirable objective of having the flat surfaces of the mask resistant to etching is achieved by the oxidation step giving rise to layers 14, 14a, of oxide and liner 13, which is resistant to oxidation, protects the mask apertures against oxidation which would otherwise take place along with the oxidizing of the flat faces of the mask. But the liner itself is readily removed chemically, for example by the etchant used in re-etch so that the necessary enlargement of the mask apertures in the re-etch step is made possible even through protective layers 14, 14a protect the flat mask surfaces against attack by the etching solution. Essentially, the same properties can be imparted to the mask if the mask blank is oxidized, developing layers similar to layers 14, 14a of FIG. 4, before the apertures are initially developed in the mask. One method of accomplishing this is to first oxidize or thermally blacken the blank before the application of resist layers 11, 11a. Then the processing proceeds in the normal fashion to develop holes in the mask like that shown in FIG. 3 except that in this case liner 13 may be omitted. This, of course, requires that the etchant used in forming the apertures be effective to attack not only the mask blank but also its oxide layers. For a steel mask an etchant for this purpose is hydrochloric acid. While this is an attractive approach to the problem solved by the present invention, it may, as a practical matter, be confined in use to flat aperture mask screens simply because it is generally necessary to anneal the apertured mask after it has been given its spherical-like shape and the annealing tends to remove the oxide layer, requiring re-oxidation which, unless protection is afforded to the walls of the apertures, would impair the re-etch ste 'I he described arrangement has most distinct advantages in the processing of color tubes which require reetching in order to enlarge the apertures of the mask. An outstanding advantage is preservation of the mechanical strength of the mask because the etch resistant surface layers protect the mask surfaces from the etchant and preserve the thickness of the mask blank. As a corollary, it is possible to use a mask blank of thinner stock, and therefore less cost, than with prior art practices wherein allowance must be made for the decrease in blank thickness as a consequence of the etchant attacking the blank surfaces during re-etch.

A further attraction is in the conservation of processing steps. In the prior process, it was necessary to strip the oxide layers from the surfaces of the mask blank prior to re-etch and then re-oxidize the mask subsequent to re-etch. With the arrangement described, a single oxidation of the mask may be sufficient and it takes place, with the apertures protected by liner 13, prior to the screening process.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. A shadow mask for use in screening a color cathode-ray tube comprising:

a mask blank of a material that is subject to attack by a predetermined etchant, having a multiplicity of apertures formed therein in accordance with a prescribed pattern of distribution, and further having on opposed surfaces thereof a coating of a material that is resistant to said etchant;

and a lining over the wall surfaces of said apertures,

having a thickness that is negligible compared to the smallest dimension of said apertures and formed of a material that is chemically removable.

2. A shadow mask in accordance with claim 1 in which the material of said mask blank is also subject to oxidation and in which said coating on at least one fiat surface of said mask is an oxide layer.

3. A shadow mask in accordance with claim 2 in which said coating on both flat surfaces of said blank is an oxide layer.

which said lining material is resistant to oxidation.

6. A shadow mask in accordance with claim 5 in which said lining material is soluble in said etchant. 

2. A shadow mask in accordance with claim 1 in which the material of said mask blank is also subject to oxidation and in which said coating on at least one flat surface of said mask is an oxide layer.
 3. A shadow mask in accordance with claim 2 in which said coating on both flat surfaces of said blank is an oxide layer.
 4. A shadow mask in accordance with claim 3 in which said blank is formed of a ferrous material and said lining material is non-ferrous and is resistant to oxidation.
 5. A shadow mask in accordance with claim 2 in which said lining material is resistant to oxidation.
 6. A shadow mask in accordance with claim 5 in which said lining material is soluble in said etchant. 